1. Field of the Invention
The present invention relates to a chargecouple type solid state imaging device (CCD type solid state imaging device). More particularly, the invention relates to a CCD type solid state imaging device used in a video camera or the like.
2. Description of the Prior Art
FIG. 2 shows a prior art CCD type solid state imaging device. This CCD type solid state imaging device comprises: a semiconductor substrate 1 having a grounded region; photoelectric conversion elements 2 arranged in a matrix pattern at the semiconductor substrate 1; vertical transfer elements 3 disposed adjacent to individual columns of the photoelectric conversion elements 2; a horizontal transfer element 4 connected to the ends of individual columns of the vertical transfer elements 3; and an output circuit 5 connected to one end of the horizontal transfer element 4. The CCD type solid state imaging device further comprises: a drain terminal 6 connected to a drain region (not shown) which is provided within the output circuit 5; an overflow drain voltage supply 8 formed in the semiconductor substrate 1; and an overflow drain voltage input terminal 7 connected to the overflow drain voltage supply 8.
A light beam incident on the surface of the semiconductor substrate 1 is converted into signal charges by the photoelectric conversion elements 2. The signal charges generated by individual photoelectric conversion elements 2 are sequentially transferred by the vertical transfer elements 3 to the horizontal transfer element 4, and are, in turn, sequentially transferred by the horizontal transfer element 4 to the output circuit 5. The output circuit 5 outputs an output signal to an output terminal T according to the amount of each signal charge received from the horizontal transfer element 4.
A prescribed drain voltage is applied on the drain terminal 6 which is connected to the output circuit 5. In general, the drain terminal 6 is electrically connected to a drain region of a MOSFET (not shown) in the output circuit 5. The potential of the drain of the MOSFET is kept constant. In addition, a gate of the MOSFET is connected to the one end of the horizontal transfer element 4, so that the potential of the gate of the MOSFET depends on the amount of each signal charge received from the horizontal transfer element 4. Therefore, the amount of each signal charge can be detected as a potential of a source of the MOSFET, or detected as a current flowing between the drain and the source.
A prescribed overflow drain voltage is applied on the overflow drain voltage input terminal 7 which is electrically connected to the semiconductor substrate 1 via the overflow drain voltage supply 8. The value of the overflow drain voltage is adjusted by an external voltage regulator unit 10 so as not to exceed the value of the drain voltage. An optimum value of the overflow drain voltage is determined so as to obtain a required level of an output signal from the output circuit 5.
For blooming suppression, the CCD type imaging device has an overflow drain structure formed in the semiconductor substrate 1. According to the overflow drain structure, overflow charges, which are generated in the photoelectric conversion elements 2 when an excessive light beam irradiates the photoelectric conversion elements 2, flow into the deep portion of the semiconductor substrate 1. Thus, blooming is prevented by the overflow drain structure. An example of such a overflow drain structure for blooming suppression, a vertical overflow drain (VOD) structure, is disclosed in detail in Japanese Patent Publication (KOKOKU) No. 59-17581.
The amount of overflow charges to be led into the semiconductor substrate 1 is adjusted by controlling the value of the overflow drain voltage which is applied by the external voltage regulator unit 10. The optimum value of the overflow drain voltage is not constant and varies from unit to unit of CCD type solid state imaging devices. Therefore, the value of the overflow drain voltage to be applied by users must be specified for each individual CCD solid state imaging device. This causes a problem that an adjustment of the value of the overflow drain voltage is required for each individual CCD type solid state imaging device when manufacturing video cameras and the like using the CCD type solid state imaging devices. In addition, it has been required that a voltage generating circuit 9 and the voltage regulator unit 10 must be externally provided for each CCD type solid state imaging device, which naturally leads to an increase in the cost of manufacturing video cameras and the like.